Authored by Piotr Plaskota
Back in April when the whole world had started to come to reality with the coronavirus pandemic, there was in fact, some good, but unrelated news that slipped through the cracks. Unfortunately for us, bad news gets much more attention in the media. Ironically, this information relates to a major pre-pandemic issue – the issue of plastic waste. Whilst environmental issues are still a key topic in today's world they have, understandably, taken the side-line. I thought however that now more than ever we could all do with a bit of good news. I have taken the opportunity to write this article about a new scientific discovery that made me feel a bit better about the world to come, despite everything going on.
In the start of April, a paper was published in Nature entitled “An engineered PET depolymerase to break down and recycle plastic bottles”. I came across this paper in an article written about it in The Guardian, and it immediately has kept a foothold in my mind. 2 The reason why this is so important is due to the amount of PET plastic waste we generate, most notably with plastic bottles. As mentioned in the abstract of the paper “estimates suggest that of the 359 million tons of plastics produced annually worldwide, 150–200 million tons accumulate in landfill or the natural environment”, with about 49% of the world’s plastic ending up landfill clearly, we have a problem. The most abundant polymer plastic is poly(ethylene terephthalate) (PET), commonly used for plastic bottles that most people are familiar with. The current problem with recycling PET is that upon recycling there is a loss of mechanical properties, so often fresh PET products need still need to be produced from petrochemical sources, spurring a continuation in plastic waste accumulation.
Figure 1: Poly(ethylene terephthalate) (PET) is the most abundant polyester plastic, with almost 70 million tons manufactured annually worldwide for use in textiles and packaging .
The reason why I am particularly optimistic upon reading this news is two-fold. Firstly, the calculated cost of the enzyme is about 4% of the ton-price of virgin PET. This means that it is an affordable technology that could be used to meaningfully lower the amount of new plastic being produced.
Secondly, Carbios, the biotechnology research and development company that developed this technology founded a Consortium in conjunction with L’Oreal now including other big names such as Nestle Waters and PepsiCo. Their four-agreement means that the likelihood of this method of recycling to reach the market, in my opinion, is high.
Figure 2: Simplified depiction of Carbios’s bio-recycling approach 
So, what is the science behind this exciting new development? PET is a polymer composed of ethylene glycol and terephthalic acid units. In order to obtain the initial units, the polymer needs to be broken down in a process called depolymerisation.
Figure 3: The depolymerisation reaction scheme for the enzymatic recycling of PET
There were already several enzymes that do depolymerisation of this kind. One of these, leaf-branch compost cutinase (LCC) was found to outperform all its other competitors, however there were problems with its stability at higher temperatures. In order to increase its thermal stability, the scientists introduced a disulphide linkage into LCC, at a key site, through mutations. These linkages are a common feature in proteins in nature, which can be thought of as staples that hold two sections together. Upon this stapling thermostability was increased but the activity of the enzyme took a hit of 28%.
Figure 4: The mutations of two residues in the LCC enabling disulphide bridge formation, increasing thermal stability 
Mutation studies of LCC were performed to see if a more active form could also be obtained. There were a few forms that had higher activity, and could also hold the disulphide bridge, called ICCG, ICCM, WCCG and WCCM. After testing with consumer-type PET plastic ICCG was selected as the best mutant enzyme for the industrial scale, on the basis of a compromise between enzyme cost and productivity.
This new enzyme caused 90% depolymerisation in 10 hrs! A highly effective method especially when compared with other enzymatic methods. A promising development, for a new, greener future.
 Tournier, V., Topham, C.M., Gilles, A. et al. An engineered PET depolymerase to break down and recycle plastic bottles. Nature 580, 216–219 (2020)
 The Guardian, https://www.theguardian.com/environment/2020/apr/08/scientists-create-mutant-enzyme-that-recycles-plastic-bottles-in-hours, Accessed 14/09/2020
 Carbios, https://carbios.fr/en/, Accessed 14/09/2020